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12.3.E: Structure and Function of Plasma Membranes (Exercises)

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  • 5.1: Components and Structure

    Among the most sophisticated functions of the plasma membrane is the ability to transmit signals by means of complex, integral proteins known as receptors. These proteins act both as receivers of extracellular inputs and as activators of intracellular processes. These membrane receptors provide extracellular attachment sites for effectors like hormones and growth factors, and they activate intracellular response cascades when their effectors are bound.

    Review Questions

    Which plasma membrane component can be either found on its surface or embedded in the membrane structure?

    1. protein
    2. cholesterol
    3. carbohydrate
    4. phospholipid


    Which characteristic of a phospholipid contributes to the fluidity of the membrane?

    1. its head
    2. cholesterol
    3. a saturated fatty acid tail
    4. double bonds in the fatty acid tail


    What is the primary function of carbohydrates attached to the exterior of cell membranes?

    1. identification of the cell
    2. flexibility of the membrane
    3. strengthening the membrane
    4. channels through membrane


    Free Response

    Why is it advantageous for the cell membrane to be fluid in nature?

    The fluid characteristic of the cell membrane allows greater flexibility to the cell than it would if the membrane were rigid. It also allows the motion of membrane components, required for some types of membrane transport.

    Why do phospholipids tend to spontaneously orient themselves into something resembling a membrane?

    The hydrophobic, nonpolar regions must align with each other in order for the structure to have minimal potential energy and, consequently, higher stability. The fatty acid tails of the phospholipids cannot mix with water, but the phosphate “head” of the molecule can. Thus, the head orients to water, and the tail to other lipids.

    5.2: Passive Transport

    Plasma membranes must allow certain substances to enter and leave a cell, and prevent some harmful materials from entering and some essential materials from leaving. In other words, plasma membranes are selectively permeable—they allow some substances to pass through, but not others. If they were to lose this selectivity, the cell would no longer be able to sustain itself, and it would be destroyed. Some cells require larger amounts of specific substances than do other cells.

    Review Questions

    Water moves via osmosis _________.

    1. throughout the cytoplasm
    2. from an area with a high concentration of other solutes to a lower one
    3. from an area with a high concentration of water to one of lower concentration
    4. from an area with a low concentration of water to one of higher concentration


    The principal force driving movement in diffusion is the __________.

    1. temperature
    2. particle size
    3. concentration gradient
    4. membrane surface area


    What problem is faced by organisms that live in fresh water?

    1. Their bodies tend to take in too much water.
    2. They have no way of controlling their tonicity.
    3. Only salt water poses problems for animals that live in it.
    4. Their bodies tend to lose too much water to their environment.


    Free Response

    Discuss why the following affect the rate of diffusion: molecular size, temperature, solution density, and the distance that must be traveled.

    Heavy molecules move more slowly than lighter ones. It takes more energy in the medium to move them along. Increasing or decreasing temperature increases or decreases the energy in the medium, affecting molecular movement. The denser a solution is, the harder it is for molecules to move through it, causing diffusion to slow down due to friction. Living cells require a steady supply of nutrients and a steady rate of waste removal. If the distance these substances need to travel is too great, diffusion cannot move nutrients and waste materials efficiently to sustain life.

    Why does water move through a membrane?

    Water moves through a membrane in osmosis because there is a concentration gradient across the membrane of solute and solvent. The solute cannot effectively move to balance the concentration on both sides of the membrane, so water moves to achieve this balance.

    Both of the regular intravenous solutions administered in medicine, normal saline and lactated Ringer’s solution, are isotonic. Why is this important?

    Injection of isotonic solutions ensures that there will be no perturbation of the osmotic balance, and no water taken from tissues or added to them from the blood.

    5.3: Active Transport

    Active transport mechanisms require the use of the cell’s energy, usually in the form of adenosine triphosphate (ATP). If a substance must move into the cell against its concentration gradient—that is, if the concentration of the substance inside the cell is greater than its concentration in the extracellular fluid (and vice versa)—the cell must use energy to move the substance. Some active transport mechanisms move small-molecular weight materials, such as ions, through the membrane.

    Review Questions

    Active transport must function continuously because __________.

    1. plasma membranes wear out
    2. not all membranes are amphiphilic
    3. facilitated transport opposes active transport
    4. diffusion is constantly moving solutes in opposite directions


    How does the sodium-potassium pump make the interior of the cell negatively charged?

    1. by expelling anions
    2. by pulling in anions
    3. by expelling more cations than are taken in
    4. by taking in and expelling an equal number of cations


    What is the combination of an electrical gradient and a concentration gradient called?

    1. potential gradient
    2. electrical potential
    3. concentration potential
    4. electrochemical gradient


    Free Response

    Where does the cell get energy for active transport processes?

    The cell harvests energy from ATP produced by its own metabolism to power active transport processes, such as the activity of pumps.

    How does the sodium-potassium pump contribute to the net negative charge of the interior of the cell?

    The sodium-potassium pump forces out three (positive) Na+ ions for every two (positive) K+ ions it pumps in, thus the cell loses a positive charge at every cycle of the pump.

    5.4: Bulk Transport

    In addition to moving small ions and molecules through the membrane, cells also need to remove and take in larger molecules and particles (see Table 5.4.1 for examples). Some cells are even capable of engulfing entire unicellular microorganisms. You might have correctly hypothesized that the uptake and release of large particles by the cell requires energy. A large particle, however, cannot pass through the membrane, even with energy supplied by the cell.

    Review Questions

    What happens to the membrane of a vesicle after exocytosis?

    1. It leaves the cell.
    2. It is disassembled by the cell.
    3. It fuses with and becomes part of the plasma membrane.
    4. It is used again in another exocytosis event.


    Which transport mechanism can bring whole cells into a cell?

    1. pinocytosis
    2. phagocytosis
    3. facilitated transport
    4. primary active transport


    In what important way does receptor-mediated endocytosis differ from phagocytosis?

    1. It transports only small amounts of fluid.
    2. It does not involve the pinching off of membrane.
    3. It brings in only a specifically targeted substance.
    4. It brings substances into the cell, while phagocytosis removes substances.


    Free Response

    Why is it important that there are different types of proteins in plasma membranes for the transport of materials into and out of a cell?

    The proteins allow a cell to select what compound will be transported, meeting the needs of the cell and not bringing in anything else.

    Why do ions have a difficult time getting through plasma membranes despite their small size?

    Ions are charged, and consequently, they are hydrophilic and cannot associate with the lipid portion of the membrane. Ions must be transported by carrier proteins or ion channels.

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